The ${}^{28}\mathrm{Si}$($p,t$)${}^{26}\mathrm{Si}$${}^{*}$($p$) reaction and implications for the astrophysical ${}^{25}\mathrm{Al}$($p,\gamma$)${}^{26}\mathrm{Si}$ reaction rate

Several resonances in ${}^{25}\mathrm{Al}$($p,\gamma$)${}^{26}\mathrm{Si}$ have been studied via the ${}^{28}\mathrm{Si}$($p,t$)${}^{26}\mathrm{Si}$ reaction. Triton energies and angular distributions were measured using a segmented annular detector array. An additional silicon detector array was used to simultaneously detect the coincident protons emitted from the decay of states in ${}^{26}\mathrm{Si}$ above the proton threshold in order to determine branching ratios. A resonance at $5927\pm 4$ keV has been experimentally confirmed as the first $\ell =0$ state above the proton threshold, with a proton branching ratio consistent with one.

Figure 1

Experimental setup for the ($p,t$) experiment. Both ORRUBA and SIDAR are symmetric (in $\varphi$) about the beam axis. The proton (“$p$”) in the diagram is the decay proton from the ${}^{26}\mathrm{Si}$, not the beam. The aluminum beam collimator is not shown.

Figure 2

Triton spectrum for the SIDAR strip at ${\theta }_{\mathrm{lab}}\approx {23}^{°}$, showing levels above the proton threshold. Possible new levels are labeled in parentheses. The level at 8166 keV was discernible from the contaminant peak at other angles; however, the peak just below 8-MeV triton energy was too weakly populated to be discernible at all angles.

Figure 3

The observed triton angular distribution populating the 7031-keV ${}^{26}\mathrm{Si}$ level. dwuck5 calculations for $0^{+}$ (green dashed) and $1^{-}$ (solid blue) assignments are shown. Assignments greater than $\ell =2$ (red dotted) were ruled out.

Figure 4

ORRUBA energy (decay protons) vs. SIDAR energy (reaction tritons) for all proton-triton coincident events. The red box, indicating the gate used, has been drawn around events associated with proton decay from excited ${}^{26}\mathrm{Si}$ levels.

Figure 5

(a) Triton energy spectrum for the SIDAR strip at ${\theta }_{\mathrm{lab}}\approx {23}^{°}$ without (black) and with (blue, lower histogram) a coincident decay proton. The rightmost triton peak (black) is the ${}^{26}\mathrm{Si}$ ground state. Energies used for the calibration are labeled with a “c” and the $E_x$ values from Refs. [16]. (b) Same as (a), but on a log scale to highlight the coincident spectrum.

Figure 6

Theoretical angular correlation curves calculated for the three anisotropically decaying levels at 6784, 7672 (both $\ell =1$), and 7875 keV ($\ell =3$), as well as the doublet at $7439+7512$ keV ($\ell =0+2$), as a function of angle between the decay proton and the ${}^{26}\mathrm{Si}$ parent.